The present invention relates to improved means for reducing the heating of a monolithic superconducting winding due to friction generated at an interface between the superconducting winding and a support structure for the winding, and more particularly to such improved means wherein the support structure comprises a low thermal diffusivity material.
As used herein, a "monolithic" superconducting winding comprises a superconducting winding structure which is substantially rigid throughout, but which is not necessarily completely solid whereby a monolithic superconducting winding can comprise cooling channels, for example. Due to the relatively rigid nature of a monolithic superconducting winding, relative vibration of a portion of the superconducting winding and the attendant heat generation therefrom is avoided. The avoidance of heat generation in, or heat transfer to, a superconducting winding is of utmost cruciality due to the superconducting nature of the winding. That is to say, a precondition of superconductivity of a superconducting winding is that windings remain at or below a very low temperature, such as 4.degree. Kelvin. Accordingly, a superconducting winding has very little capacity for absorbing heat.
Where the support structure for a superconducting winding is subjected to a rapidly varying magnetic field, eddy currents and heating therefrom can be generated in the support structure for the superconducting winding. In order to limit eddy current heating of support structure for such a superconducting winding, the support structure material must comprise a material with a high electrical resistivity whereby eddy current heating therein is limited to a tolerable level: that is to say, a level in accordance with the capacity for the superconducting winding to absorb heat without losing the property of superconductivity. It is typical for support structure with such high electrical resistivity to also have the property of low thermal diffusivity, or, in other words, the inability to absorb significant amounts of heat.
Relative sliding between an external surface of a monolithic superconducting winding and a support structure having low thermal diffusivity presents an acute problem. The frictional heat generated from the relative sliding motion is not able to be effectively absorbed by the support structure. As a consequence to this fact, frictional heat caused by the relative sliding motion is known to have been transferred to a superconducting winding with a resultant loss of superconductivity thereof.
One prior art means for minimizing the foregoing friction heating problem has been to hold the support structure for a superconducting winding under high compression, such as 5,000 psi, against the winding. However, relative sliding motion between the support structure and the superconducting winding, probably localized in nature, is known to have resulted in heating of the superconducting winding to a point where it loses the property of superconductivity.